In total knee replacement (TKR) surgery, a surgeon typically affixes two prosthetic components to the patient's bone structure—a first to the patient's femur and a second to the patient's tibia. These components are typically known as the femoral component and the tibial component, respectively. The femoral component is placed on a patient's distal femur after appropriate resection of the femur. The femoral component is usually metallic, having a highly polished outer condylar articulating surface, which is commonly J-shaped when viewed from a medial or lateral side.
A common type of tibial component uses a tray or plateau that generally conforms to the patient's resected proximal tibia. The tibial component also usually includes a stem that extends into a surgically formed opening in the patient's intramedullary canal.
A plastic or polymeric (often ultra high molecular weight polyethylene) insert or bearing fits between the tray of the tibial component and the femoral component. This insert provides a surface against which the femoral component condylar portion articulates, i.e., moves in gross motion corresponding generally to the motion of the femur relative to the tibia. One type of design is a posterior-stabilized design in which the insert includes a post that fits within box walls and a posterior cam of the femoral component, with the posterior cam stabilizing the implant against anterior tibial sliding when the knee is flexed.
A common complaint of TKR patients is that the replaced knee does not function like a normal knee or does not “feel normal.” The replaced knee does not achieve normal knee kinematics or motion and generally has a more limited range of motion than a normal knee. Currently available designs typically produce kinematics different than the normal knee during gait due to the complex nature of the knee joint and the motion of the femur and tibia relative to one another during flexion and extension. For example, it is known that, in addition to rotating about a generally horizontal axis during flexion and extension, the tibia also rotates about its longitudinal axis. Such longitudinal rotation is typically referred to as either external or internal rotation, depending on whether reference is being made to the femur or tibia, respectively.
Few currently available posterior-stabilized designs achieve this longitudinal rotation. Most currently available designs provide a limited space between the insert post and the box walls of the femoral component, to increase insert post strength and contact area between the femoral posterior cam and the insert post. The limited space between the insert post and the box walls would, in most cases, impede the ability of the post to rotate longitudinally—that is, either by external or internal rotation within the box wall, as would be the natural motion of a healthy knee. Because few designs achieve this longitudinal rotation, this impediment is not generally recognized.
FIG. 1A and FIG. 1B depict top views of a prior art knee system with a standard tibial post fitted inside a femoral box wall. The post 400 is seated within the box wall 200 to form the post/box-wall configuration 450. FIG. 1A depicts the configuration 450 with arrangement of the post 400 and the box wall 200 during extension, where the walls 402-408 of the post are situated in parallel with walls 202-208 of the box 200. As shown, the walls of the post 400 are disposed near but not in direct contact with the box walls from the femoral component 200. The separation is depicted by the distance “C.” In many standard knee systems, the distance “C” is approximately 0.5-1.5 mm, allowing a small space around the perimeter of the post. When flexion occurs, axial tibial and femoral rotation is affected because a corner of the post contacts the sidewalls of the femoral box wall. FIG. 1B depicts the posterior lateral corner 420 butting against the box wall 206 and the anterior medial corner 422 butting against the box wall 204 in the prior art system during flexion as a result of tibiofemoral rotation in the direction of arrow “TFR.”
Constructing a total knee prosthesis which replicates the kinematics of a natural knee has been an on-going challenge in the orthopedic field. Several attempts have been made and are well known in the prior art, including those shown in U.S. Pat. Nos. 6,264,697, 6,325,828, US2005/0143832, and US2008/0119940. Other systems have been designed to allow rotation by altering the surfaces of the box wall. However, modification of the box wall can reduce the effectiveness of the box in constraining both varus and valgus motion during flexion with tibial inserts that have a wider, constrained post. Other systems and approaches that attempt to more closely replicate the structure and function of the human knee produce modifications to the post that narrow its width. Those implementations, however, reduce the contact surface between the post and the posterior cam surface of the femoral component, which can lead to increased post deformation or wear in the cam and post contact region, and can reduce the amount of material in the post, thereby reducing its strength.
Existing designs leave room for improvement in simulating the structure and operation of actual knee joints.